Field of the Invention
The invention relates to a process for the production of an insulating component for a high temperature fuel cell, and to a high temperature fuel cell.
It is known that, during the electrolysis of water, water molecules are decomposed by electric current into hydrogen (H.sub.2) and oxygen (O.sub.2). In a fuel cell, that process takes place in reverse. Electrical current is produced with high efficiency through electrochemical combination of hydrogen (H.sub.2) and oxygen (O.sub.2) to form water. When pure hydrogen (H.sub.2) is used as the combustible gas, the process takes place without the emission of pollutants and carbon dioxide (CO.sub.2). Even with a technical combustible gas, for example natural gas or coal gas, and with air (which may also be enriched with oxygen (O.sub.2)) instead of pure oxygen (O.sub.2), a fuel cell produces considerably less pollutants and less carbon dioxide (CO.sub.2) than other forms of energy production which operate with fossil energy sources. The technical implementation of that principle has given rise to a variety of solutions, specifically with different electrolytes and with operating temperatures of between 80.degree. C. and 1000.degree. C.
Fuel cells are classified as low, medium and high temperature fuel cells according to their operating temperature, and they in turn differ over a variety of technical embodiments.
In high temperature fuel cell stacks (a fuel cell stack is also abbreviated as "stack" in the specialist literature) composed of a large number of high temperature fuel cells, at least one protective layer, a contact layer, an electrolyte/electrode unit, a further contact layer, and a further interconnecting conducting plate, etc. are disposed in that order under an upper interconnecting conducting plate which covers the high temperature fuel cell stack.
In that case, the electrolyte/electrode unit includes two electrodes and a solid electrolyte which is constructed in the form of a membrane and is disposed in between the two electrodes. That being the case, an electrolyte/electrode unit lying between two neighboring interconnecting conducting plates respectively forms a high temperature fuel cell, with the contact layers bearing directly on both sides of the electrolyte/electrode unit. Both sides of each of the two interconnecting conducting plates which bear on the contact layers also belong to the high temperature fuel cell. That and other types of fuel cells are, for example, known from the "Fuel Cell Handbook" by A. J. Appleby and F. R. Foulkes, 1989, Pages 440 to 454.
In regions where there is no electrolyte/electrode unit provided between neighboring interconnecting conducting plates, it is necessary for the interconnecting conducting plates to be electrically insulated from one another. In order to provide partial electrical insulation of neighboring interconnecting conducting plates from one another, an insulating component is provided which has the form of a frame. Feedthroughs are provided in the insulating component for gaseous working media for the electrolyte/electrode unit. Neighboring feedthroughs (which feed different working media) must be isolated in a gas-tight manner from one another. Further, the insulating component must ensure that no working medium reaches the outside of the high temperature fuel cell, i.e. for example the environment. The material of the insulating component must therefore be impermeable to gases, and at the same time should not exhibit any electrical conductivity.
The production of an insulating component of that type for a high temperature fuel cell proves highly elaborate. In one production process known from the prior art, the insulating component is applied directly to the interconnecting conducting plate (i.e. the components to be joined together, in that case the interconnecting conducting plates, are directly involved in the production process). The composition in the surface of the interconnecting conducting plate is partially altered, at least in the short term. Further, mechanical damage to the surface of the interconnecting conducting plate may occur during the production process.
In a further process known from the prior art for producing the insulating component (wherein the insulating component is formed for the most part of a ceramic material) the insulating component is produced by the use of ceramic manufacturing techniques (for example pressing and sintering plates, adjusting the thicknesses of the components by grinding, structuring by using a laser, etc.). In that process, considerably more material is used for the processing than is ultimately needed for the finished insulating component. Both processes prove very involved and cost-intensive.